A wireless communication system is a complex network of systems and elements. Typical systems and elements include (1) a radio link to mobile stations (e.g., a cellular telephone or a subscriber equipment used to access the wireless communication system), which is usually provided by at least one and typically several base stations, (2) communication links between the base stations, (3) a controller, typically one or more base station controllers or centralized base station controllers (BSC/CBSC), to control communication between and to manage the operation and interaction of the base stations, (4) a switching system, typically including a mobile switching center (MSC), to perform call processing within the system, and (5) a link to the land line, i.e., the public switch telephone network (PSTN) or the integrated services digital network (ISDN).
A base station subsystem (BSS) or a radio access network (RAN), which typically includes one or more base station controllers and a plurality of base stations, provides all of the radio-related functions. The base station controller provides all the control functions and physical links between the switching system and the base stations. The base station controller is also a high-capacity switch that provides functions such as handover, cell configuration, and control of radio frequency (RF) power in the base stations.
The base station handles the radio interface to the mobile station. The base station includes the radio equipment (transceivers, antennas, amplifiers, etc.) needed to service each communication cell in the system. A group of base stations is controlled by a base station controller. Thus, the base station controller operates in conjunction with the base station as part of the base station subsystem to provide the mobile station with real-time voice, data, and multimedia services (e.g., a call).
To ensure that all mobile stations receive proper communication services, the base station typically provides power control information. To illustrate this concept, mobile stations serviced by the base station may be anywhere in the communication cell from right under the antenna of the base station to a couple of miles away from the base station. If all of the mobile stations used the same amount of transmission power (i.e., radio frequency (RF) output power), a mobile station proximate to the base station may cause interference to a mobile station distant from the base station. To mitigate this problem, the base station provides each of the mobile stations with power control information so that the mobile stations may optimally control their transmission power and all signals arrive at the base station at substantially equal power. For example, the base station may instruct a mobile station to operate at a reduced transmission power to avoid causing interference to other mobile stations. In particular, the mobile station may vary its transmission power from a few tens of nanowatts up to the order of one watt (W). To reduce transmission power, the gain of the mobile station may be attenuated by adjusting the bias level of the mobile station (e.g., reducing control voltage of the mobile station). Accordingly, the mobile station demands less supply current which, in turn, reduces the third-order intercept point (IP3) level of the mobile station. In particular, the IP3 level indicates the linearity of an RF signal (i.e., distortion level of the signal) from the mobile station. Therefore, higher IP3 level may result in a more distortion-free RF signal from the mobile station but may also require more supply current (i.e., result in greater power consumption by the mobile station).
Typically, the supply current of a mobile station is constant (i.e., the input power is constant). However, the third-order intercept point (IP3) level may decrease as the transmission power is reduced. Thus, unless the input power of the mobile station is appropriately reduced as well, inter-modulation distortion may be generated during reduction of transmission power because of an insufficient IP3 level for a given amount of input power. In particular, inter-modulation distortion may cause RF energy to overflow into adjacent and/or alternate channels of a frequency band. That is, the RF energy may exceed the acceptable level specified by a wireless communication protocol that is allowed in the frequency band. As a result, spurious RF emissions and degradation in signal quality may occur during reduction of transmission power.
Further, the trend in the wireless communication industry is not only to provide small hand-held portable devices but also to provide additional user features and functionality. Thus, one aspect of designing a wireless communication system is to optimize the resources available to the mobile station. That is, one method of improving the availability of resources is to reduce the amount of power consumed by a transmitting unit of the mobile station. By reducing power consumption of a mobile station, transmission time may be extended and/or smaller capacity and size batteries may be used.
Therefore, a need exist to control transmission power associated with a transmitting unit of a mobile station to improve transmission efficiency during power reduction and to optimize communication resources of a wireless communication system.